Eiruvin 3:3-4

Eiruvin 3:3

If a person put his eiruv techumin in a tree, higher than ten handbreadths (about 30”), it is invalid; lower than this, it is valid. If he put it in a pit, it is valid even if it is 100 cubits deep (approximately 150’). If he put the eiruv on top of a reed or a pole that was detached from the ground and then re-inserted, it is valid even if it is 100 cubits high. If he put it in a cabinet and lost the key, the eiruv is valid. Rabbi Eliezer says that the eiruv in such a case is invalid unless he knows that the key is in its place.

Eiruvin 3:4

If the eiruv techumin rolled outside the Shabbos limit, or a pile of stones fell on it, or if it burned up, or if it was terumah and got rendered ritually unclean – if any of these things happened before Shabbos started, the eiruv is invalid. If it happened after Shabbos began, the eiruv is valid. If it is unknown when it occurred, Rabbi Meir and Rabbi Yehuda refer to the situation as “a donkey driver and a camel driver.” [A donkey is driven from behind and a camel is driven from in front. The meaning is that maybe the eiruv took effect and his personal Shabbos boundary is area A. But maybe the eiruv did not take effect and his personal Shabbos boundary is area B. Because of this doubt, he is limited to the area that is common to both A and B, which is basically from his home in the direction of the eiruv.] Rabbi Yosi and Rabbi Shimon say that a doubtful eiruv is valid. Rabbi Yosi said that Avtulmos reported in the name of five elders that a doubtful eiruv is valid.